`discoverBlocks` helper doesn't work against using the REST API on a local real plone backend

[JeffersonBledsoe]

A plone backend API call running on localhost (e.g. http://localhost:8080/Plone/++api++/) returns the root of the site with the site ID (i.e. all @id values start with http://localhost:8080/Plone). The discover-blocks function makes the assumption that any paths returned by the @id will be relative to the root of the API URL. However, because the @id contains /Plone and the apiURL ends with /Plone we end up with API calls being to http://localhost:8080/Plone/++api++/Plone